THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



603 



Fig. 613. — Isoetes lacustris. Development of the male gametophyte showing diifer- 

 entiation of prothallus cell and antheridial cell, the latter forming a wall and 

 antherozoid mother cells. Note that the prothallial cell does not disorganize as in 

 Selaginella. {From Smith after Liebig.) 



The Female Gametophyte 



The megaspores are nearly globular, but the three converging ridges on 

 one side are very prominent and form the angles of a flattened pyramid. It is 

 along these ridges that the spore dehisces. 



At the time of shedding the protoplasm fills the v^hole cavity of the spore, 

 and at no time is there a large central vacuole, as there is in Selaginella. 

 There are large numbers of both oil drops and starch grains present. 



The primary spore nucleus lies at the apex of the protoplast, and its 

 first divisions are free, without any cell-wall formation. This continues 

 till about fifty nuclei have been formed, most of which are congregated at the 

 apical end with only a few elsewhere. Cell-wall formation then begins 

 simultaneously right across the spore apex. It starts with the formation of 

 incompletely enclosed areolae, which are subsequently closed by end walls. 

 Cell formation spreads along the periphery towards the base of the spore, 

 and only finally is the central mass divided up into uninucleate cells (Fig. 615). 

 The spore wall now opens from the apex along the ridges, disclosing the 

 cellular surface of the prothallus. Only one archegonium is at first produced, 

 and if this is fertilized no other appears, but if fertilization fails a succession 

 of archegonia, up to twenty or thirty, may be formed. The archegonium is 

 developed from one superficial cell, which enlarges at an early stage. This 

 divides horizontallv into two, the upper of which forms a four-tiered neck ; 

 the lower cell again divides into a single canal cell, below the neck, and a 

 lowermost cell which itself divides to form the ventral canal cell, on top, 

 and the oosphere below. The neck scarcely rises above the prothallus 

 surface. 



